1. Field of the Invention
This invention relates to methods of metal forming and more particularly to a method for sizing powder metal parts.
2. Prior Art
Fabrication of powder metal parts is well known. This fabrication technique allows parts having intricate configurations to be produced without the need for expensive machining and other such operations.
A mixture of metal powders is loaded into a die having a shape of the part. The metal powders are then compacted with a punch into a green briquette having a substantially finalized shape. Next, the part is sintered in a protective atmosphere wherein the powders are heated and joined into a solid mass. Density of such parts may typically vary from 75 to 95 percent.
Parts having a reduced density may be infiltrated with a further material, for example copper, where the part is to be used as a bearing. Lastly, the part may be tumbled to remove burrs.
Where the part is to have an inner cavity, the cavity may be formed with the die during compacting of the powders. Without additional processing, the cavity can be so made within a dimensional tolerance of .+-.0.005, for example. Where the dimensional tolerance of the cavity must be .+-.0.001, an additional operation is required. To obtain this improved dimensional control, the cavity may be cold balled wherein a surface of the cavity is impacted by a tool having an outer dimension substantially equal to the cavity dimension sought. Cold balling is regulated to produce stress substantially equal to the yield point of the cavity wall to produce some permanent deformation. However, because portions of the wall are subject to stresses less than the yield point, the wall is in part merely deflected and returns to its prior configuration. Cold balling, while heretofore considered the most economical means for sizing the part cavity to the dimension sought, nevertheless produces a substantial number of out-of-tolerance parts.